Choice of Vcc and LDO Regulator

[Papabravo]

I'd like to know what values of Vcc people are choosing for low pwer projects. Second, once a value of Vcc has been chosen, what voltage regulators are being used. Presumeably these are of the LDO variety. The reason I ask is because my first choice led me to parts that are only available in packages too small to handle, let alone solder, without the benfit of a microscope. My eyes are just not as good as they once were.

 

[Nigel Goodwin]

That's really FAR too vague a question?, but for a start why not design so you don't require a regulator? (assuming it's possible?).

 

[Papabravo]

I see your point, but assuming for a moment that I have a reason for selecting a particular value and therefore selecting a fixed as opposed to an adjustable regulator, I'd like to know how others have resolved the issue.

At least one reason for selecting a fixed Vcc is to interface to other systems which have requirements for Vih and Vil. Another reason is to have a fixed Vcc which also serves as the A/D reference. Another reason is to have multiple options for input voltage which exceed the maximum Vcc ratings of the part. I could go on but, I think I've clarified the issue sufficiently.

 

[mvs sarma]

Hi Papabravo,
I feel at my 61+, the it is reasonable that we go for normal components like 7805 or 78L05 as regulator and standard DIP packages untill we complete the project to a workable stage. if later bulk assy is required and SMDs are to be used as , so called state of art , or for minaturization, it would be better to train a junior and get it done then we suffer the issue-- for eample, i am now soldering a DDS chip (ssop28 from analog devices) in its position. Because i suffered -ve sight earlier and now the readings are near ZERO, but for sperical power being on negative side, I am able to manage. May be if your eyesight was very normal earlier you are now likely to have +ve power glasses.
as far as regulators are concerned, whether you use 3.3V or 5V as Vcc, depending on the Controller used, LOw drop ones caould be used if equipment is run on cells (6V or 4.5V) or even 9V. as at low power consumption the entire capacity of cells can be made use of. Otherwise in case of normal 5V regulator you need to maintain a minimum of 3V difference between input and output. this is well possible in Adopter (wall wart) based power supplies.
All the best.

 

[Nigel Goodwin]

I see your point, but assuming for a moment that I have a reason for selecting a particular value and therefore selecting a fixed as opposed to an adjustable regulator, I'd like to know how others have resolved the issue.

At least one reason for selecting a fixed Vcc is to interface to other systems which have requirements for Vih and Vil. Another reason is to have a fixed Vcc which also serves as the A/D reference. Another reason is to have multiple options for input voltage which exceed the maximum Vcc ratings of the part. I could go on but, I think I've clarified the issue sufficiently.

Not really, you're looking for a general answer to every single possible application out here - and that isn't going to happen!. You deal with specific design problems in specific ways, and it's really not a good idea to use Vcc as a reference for an A2D, use a precision voltage reference instead.

Personally I usually use either 7805 or 78L05, or for battery applications I'd probably use AA batteries, and suitable devices (like L series PIC's, or 3 x 1.5V AA, or 4 x 1.2V AA).

 

[Papabravo]

....
Personally I usually use either 7805 or 78L05, or for battery applications I'd probably use AA batteries, and suitable devices (like L series PIC's, or 3 x 1.5V AA, or 4 x 1.2V AA).

Thanks for that -- so you use +5V or batteries. Fair enough.

 

[Hero999]

4 x 1.2V AA).

I wouldn't use 4 1.2V NiMH batteries because they can be >1.5V each when taken hot off the charger giving you >6V which is too much for the average PIC to stand.

 

[Papabravo]

So a regulator just might be handy in this case? Eh?

 

[Nigel Goodwin]

I wouldn't use 4 1.2V NiMH batteries because they can be >1.5V each when taken hot off the charger giving you >6V which is too much for the average PIC to stand.

It's not a problem, there are a great many commercially produced PIC based units running off 4 x NiCd or 4 x NiMh - you can get too obsessive about this!

 

[usmankhan]

it is a good circuit

 

[bananasiong]

It's not a problem, there are a great many commercially produced PIC based units running off 4 x NiCd or 4 x NiMh - you can get too obsessive about this!

4 AA batteries running the PIC without a voltage regulator? Won't it be higher than 5.5 V? If with a voltage regulator, the voltage of the batteries may drop quickly.

it is a good circuit

Where did you see the good circuit? -_-"

 

[Nigel Goodwin]

4 AA batteries running the PIC without a voltage regulator? Won't it be higher than 5.5 V? If with a voltage regulator, the voltage of the batteries may drop quickly.

4 x NiCd or NiMh as I said, would give a nominal 4.8V - PIC's don't self destruct at 5.1V - it's a perfectly common method of operation, and is used commercially extensively.

 

[Hero999]

Won't PICs be damaged by 6V though?

 

[Nigel Goodwin]

Won't PICs be damaged by 6V though?

Three normal batteries is only 4.8V as are four NiCd's, either work perfectly well, even four normal batteries (6V) doesn't seem to cause any damage, although I wouldn't recommend it. Don't get too concerned about short term maximum peak voltages from the batteries.

 

[mneary]

Most older pics are specified to withstand 6.0V and some of the new ones can handle 7.5 volts. They aren't guaranteed to run properly above 5.5, but they shouldn't be permanently harmed.

 

[Oznog]

Well be aware that having too low of a voltage at worst means it won't run. Going to high, even briefly, often causes a latch state. In a latch state the device won't run and might draw excessive current and be permanently damaged. The latch state will persist until the power is completely removed- not just until the Vdd drops back into a specified range.

 

[Hero999]

If you're worried about large currents then a polyfuse could help, there again the lowest polyfuse rating I've seen is 100mA which is still more than enough to nuke a PIC.